The present invention relates generally to a keyboard element and, more particularly, to a double-domed elastomeric keyboard element.
U.S. Pat. No. 4,127,758 discloses a domed elastomeric keyboard element. There is also known a double-domed elastomeric keyboard element in which a conductive, elastomeric moving contact member is located inside the keyboard element housing and joined thereto at the junction of the two domes. When the keyboard element is actuated, the moving contact member shifts downwardly to bridge a pair of conductors on a printed circuit board serving as a support for the keyboard element.
In the aforementioned prior art double-domed keyboard element, the moving contact member is formed as a ring. When pressure is applied to the upper dome of the element, the larger lower dome is initially deformed. After a certain partial travel of the keyboard element the lower dome collapses whereupon the contact ring engages the conductors on the printed circuit board below the element. The force of application is determined by the tension of the material of the upper dome. This force cannot be greater than the maximum actuating force permitted for the respective type of pushbutton keys. In the case of alpha-numeric keyboards, this force ranges from 50-100 g. In order to assure a reliable contact being made with the conductors on the printed circuit board there is required an expensive surface treatment of the contact member of the keyboard element.
The upper dome permits a certain over travel which is necessary for extending the contact closing period. However, collapsing of the upper dome must be avoided. This can be achieved by restricting the travel of the actuating plunger operating the keyboard element. The contact pressure, in fact, reduces itself by the sum of the return forces of the collapsed domes acting in opposition to the applied force. Moreover, the additional flexing considerably reduces the cycle life of the keyboard element.
It is the object of the present invention to improve and optimize the function of a double-domed keyboard element.